Chlorinated pyridines and more particularly, symmetrical 2,3,5,6-tetrachloropyridine (hereinafter sym-tetrachloropyridine or s-TCP) are compounds highly valuable as intermediates in the preparation of numerous products having application as agricultural chemicals. The compounds are also directly useful as insecticides or herbicides.
Various methods are known for the preparation of chlorinated pyridines, particularly sym-tetrachloropyridine. These known synthetic methods include the zinc reduction of pentachloropyridine (hereinafter PCP) in U.S. Pat. No. 3,993,654. High conversion of PCP with commercially acceptable selectivity to s-TCP is reported, however, the process consumes large quantities of the expensive zinc and produces a zinc chloride solution that has a potential disposal problem. In addition, the reaction mixture is extremely corrosive.
Several approaches have been attempted which avoid zinc consumption in the catalytic reduction of PCP; however, these processes lack the desired selectivity to s-TCP versus unsymmetrical tetrachloropyridine (hereinafter unsym-TCP) isomers. Generally, catalytic reduction prefers the 2-chlorine, giving primarily the unsym-TCP isomer.
It is desirable to avoid the problems in the prior art processes and yet produce s-TCP in high yields and purity. As will be realized, the tetrachloropyridine product, constituting the subject matter of this invention, minimizes the need for separation techniques which have heretofore been necessary for obtaining s-TCP free from the undesired unsym-TCP isomers. The prior art does not recognize that nearly 100 percent selective replacement of bromine for chlorine in the 4-position of PCP can be obtained under conditions described herein. Neither has it heretofore been recognized that this process for selective production of 4-bromo-2,3,5,6-tetrachloropyridine can be combined with a process for catalytic reduction which prefers bromide over chloride under sufficiently mild conditions to give substantially pure s-TCP.